What is Euclid?

Euclid is an ESA medium class space mission selected for launch in 2020 in the Cosmic Vision 2015-2025 programme (ESA PR selection ). The Euclid mission aims at understanding why the expansion of the Universe is accelerating and what is the very nature of the source of this acceleration that physicists refer to as dark energy.

Together with dark matter, dark energy dominates the matter-energy content of the Universe. Both are mysterious and of unknown nature but control its past, present and future evolution. Euclid will explore how the Universe evolved over the past 10 billion years to address the following key fun­da­mental cosmological questions:

– is dark energy merely a cosmological constant, as first discussed by Einstein, or

– is it a new kind of field that evolves dynamically with the expansion of the universe?

– alternatively, is dark energy instead a manifestation of a break-down of General Relativity and deviations from the law of gravity?

– what are the nature and properties of dark matter?

– what are the initial conditions which seed the formation of cosmic structure?

The imprints of dark energy and gravity will be tracked by using two complementary cosmological probes to capture signatures of the expansion rate and the growth of cosmic structures in the Universe: Weak gravitational Lensing and Galaxy Clustering (Baryonic Acoustic Oscillations and Redshift Space Distortion). These two probes will be complemented by independent information also derived from Euclid data on clusters of galaxies and the Integrated Sachs-Wolf effect.

To achieve the Euclid’s quest a satellite is under construction equipped with a 1.2 m telescope made by Astrium that feeds 2 instruments, built by the Euclid Consortium : a high quality panoramic visible imager (VIS), a near infrared 3-filter (Y, J and H) photometer and a slitless spectrograph (NISP). These instruments will explore the expansion history of the Universe and the evolution of cosmic structures by measuring the modification of shapes of galaxies induced by gravitational lensing effects of dark matter, and the 3-dimension distribution of structures from spectroscopic red­shifts of galaxies and clusters of galaxies.

The satellite will be launched by a Soyuz ST-2.1B rocket and transferred to the L2 Lagrange point for a 6 years mission. Euclid will observe 15,000 deg2 of the darkest sky that is free of contamination by light emissions from our Galaxy and our Solar System (see the ESA Euclid mission summary ).